Magnetoresistive Random Access Memory (MRAM) has a read function based on a tunneling magnetoresistive (TMR) effect in a magnetic tunnel junction (MTJ) stack of layers wherein a tunnel barrier is formed between a free layer and a reference layer. The free layer serves as a sensing layer by switching the direction of its magnetic moment in response to external fields (media field) while the reference layer has a fixed magnetic moment. The electrical resistance through the tunnel barrier (insulator layer) varies with the relative orientation of the free layer moment compared with the reference layer moment and thereby provides an electrical signal that is representative of the magnetic state in the free layer. In MRAM, the MTJ is formed between a top conductor (electrode) and bottom conductor. When a current is passed through the MTJ, a lower resistance is detected when the magnetization directions of the free and reference layers are parallel (“0” memory state), and a higher resistance is noted when they are antiparallel (“1” memory state). The TMR ratio is dR/R where R is the minimum resistance of the MTJ, and dR is the difference between the lower and higher resistance values. The tunnel barrier is typically about 10 Angstroms thick so that a current through the tunnel barrier can be established by a quantum mechanical tunneling of conduction electrons. When the MTJ is a sensor in a magnetic read head that is used as the read-back element in hard disk drives (HDD), a higher TMR ratio allows a faster read out of the sense current.
MTJ elements wherein one or both of the free layer and reference layer have perpendicular magnetic anisotropy (PMA) are preferred over their counterparts that employ in-plane anisotropy because the former has an advantage in a lower writing current for the same thermal stability, and better scalability for higher packing density which is one of the key challenges for future MRAM applications. The ability to maintain free layer magnetization direction during an idle period is called data retention or thermal stability. A MTJ typically has a bottommost layer called a seed layer, which is a non-magnetic metal, or alloy that is employed to induce or enhance PMA in overlying magnetic layers, and to improve film thickness uniformity in the tunnel barrier.
A MTJ with magnetic layers having PMA that is induced or enhanced with a seed layer is found in read head sensors, thermally assisted magnetic recording (TAMR), and microwave assisted magnetic recording (MAMR) devices. MAMR is described by J-G. Zhu et al. in “Microwave Assisted Magnetic Recording”, IEEE Trans. Magn., vol. 44, pp. 125-131 (2008). Spin transfer (spin torque) devices in MRAM and in MAMR writers are based on a spin-transfer effect that arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current passes through a magnetic multilayer in a CPP (current perpendicular to plane) configuration, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic layer, or drive the magnetization into stable dynamics, if the current density is sufficiently high.
According to one MAMR design that features a spin torque oscillator (STO) device between a main pole and a write shield (not shown), the STO has a seed layer contacting the main pole, and a spin polarization (SP) layer, tunnel barrier, and oscillation layer (OL) sequentially formed on the seed layer. A direct current or pulsed current flowing through the STO stack from the main pole to the write shield is converted to spin polarized current by the SP layer and interacts with the OL to cause the latter to oscillate with a large angle, and a frequency that generates a rf field on a nearby magnetic medium thereby assisting a magnetic field from the main pole to switch a magnetic bit during a write process. The seed layer may be advantageously used to enhance PMA in the SP layer and overcome the perpendicular demagnetization field within the SP layer and enable the SP layer to spin polarize the current directed to the OL.
Referring to FIG. 1a, when patterning a plurality of MTJs in magnetic devices, an etching step with ions 30 is employed to transfer a mask shape in a photoresist layer 40 through a MTJ stack with an arbitrary number of layers designated as 2-4 formed on a substrate 1 that may be a bottom electrode or main pole. As shown in FIG. 1b after the photoresist layer is removed, the resulting sidewall 5s in the MTJ 5a preferably stops on a top surface 1t of the substrate. The etch process is monitored by a method such as secondary ion mass spectrometry (SIMS) that detects the secondary ions and neutral species produced in the etch chamber. As each layer 2-4 is removed by the etch process, ions that are characteristic of a ferromagnetic layer or of a seed layer, for example, are identified by a particular m/e signal where m is the mass of the ion and e is the charge.
Referring to FIG. 2, a curve is generated on a plot of time vs. ion concentration for each layer 2-4 by a SIMS end point detector or the like. An end point signal is used to determine when the etch process has reached the top surface 1t of substrate 1. The end point e1 of the etch may be set where there is a signal decrease d from a peak intensity to x % of the peak intensity for layer 2 which is typically a seed layer. Ideally, at time e1 there is no seed layer remaining in regions not protected by the photoresist layer, and the top surface 1t remains unetched. Unfortunately, the end point signal is often not reliable because the seed layer signal does not have a strong peak intensity that makes a decrease to the x % mark at point e1 difficult to determine. As a result, the etch process easily goes to an overetch condition (right of e1) and undesirably removes a top portion of the substrate, or does not completely remove layer 2 (underetch condition to left of e1). In either case, magnetic performance of the MTJ is degraded and device reliability suffers.
As MTJ devices decrease in diameter to 50 nm or below in order to satisfy higher density requirements, a more precise control is needed for the etch process that forms MTJ sidewalls and defines the shape of the device. Currently, there is a significant signal to noise (S/N) ratio in the detection method that determines the etching end point. As a result, there is often an overetch or underetch condition of up to +/−20 Angstroms in establishing a bottom 5b of sidewall 5s in FIG. 1b that causes wafer to wafer variability in device performance. An improved film stack is desired to provide better process control during fabrication of magnetic devices described herein.